pgsql: Minor editing for README-SSI.

Minor editing for README-SSI.

Fix some grammatical issues, try to clarify a couple of proofs, make the
terminology more consistent.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a3290f655e12f43943bdabc9fd91477316af24a7

Modified Files
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src/backend/storage/lmgr/README-SSI | 116 +++++++++++++++++++---------------
1 files changed, 65 insertions(+), 51 deletions(-)